This invention relates to a circuit for preventing predetermined command types to a system control unit of a data processing system from being locked out, and more particularly, to a signal generating circuit for preventing the lock-out of high priority requests to a system control unit having multiple command level conditional rotational servicing priority hierarchy.
System control units of present systems have included a port priority based on port group, and as a result, a blocked port may be bypassed. Blocked ports can occur especially in tandem configurations of data processing systems whereby two central processor units (CPUs) are making requests to each of two corresponding system controller units (SCUs) and two input/output (I/O) units are each connected to both SCUs. The present invention is incorporated in a system controller with a plurality of command types having a predetermined priority level. Port servicing of the system controller which incorporates the present invention is based upon two levels of commands, high and low, the high level commands having priority over low level commands. Each level is independent of the other. The priority within the level is rotated upon servicing a port, provided that a higher priority port within the level does not have a service request pending which is blocked due to some necessary portion of the system being unavailable. If it is determined that a lower high priority request has been serviced a predetermined number of times over a pending higher high priority request, the circuit of the present invention then generates a control signal such that the system controller inhibits further requests from the equipments (the CPUs and I/O units attached thereto) from being processed by the SCU. An individual port user may change its command level within the established system requirements if it is not selected in a predetermined select interval.